1. Field of the Invention
The present invention relates generally to an electron cyclotron resonance heated plasma source for producing a plasma stream for use in applications such as the treatment of specimens such as VLSI wafers, by processes including chemical vapor deposition, etching, and cleaning, and more particularly, to a method and apparatus employing a coaxial resonant multiport microwave applicator for use with an electron cyclotron resonance heated plasma source.
2. Description of Related Art
The present invention is based upon generation of cold plasma by electron cyclotron resonance heating in order to develop various desired characteristics within the plasma. More specifically, the present invention and related prior art are based upon the physical conditions governing the steady-state values of plasma density and composition, as well as the electron and ion temperatures, as determined by the applied microwave power, the ambient gas pressure and composition, and details of the magnetic field configuration, and the means of coupling the microwave power to the plasma. These conditions can be interpreted in terms of the processes responsible for particle and power balance for each of the coupled species; namely, electrons, ions, and neutral gas atoms.
Even in idealized point-model approximations of somewhat complete and realistic transport models of the plasma, the interrelated values of the basic plasma parameters characterizing such systems demonstrate several key dependencies. These dependencies identify some of the fundamental obstacles that make it difficult in practice to achieve large volumes of quiescent, homogeneous, low-temperature plasma containing the desired concentrations of reacting species believed to be essential in various applications, for example, the treatment of specimens such as wafers used for very large scale integrated (VLSI) circuits. The applications may include chemical vapor deposition, etching, and cleaning, and more specifically, plasma assisted processing of semiconductor devices with submicron feature sizes.
Basic problems in forming suitable plasmas are noted in the following prior art references. The following discussion of the prior art and the following description of the embodiments of the present invention that is partially based upon a comparison with the prior art, are intended to demonstrate novelty in the method and apparatus of the present invention for overcoming disadvantages and obstacles, such as those generally referred to in the prior art.
In the semiconductor processing applications noted above, plasma sources employing electron cyclotron resonance (ECR) heating comprise an emerging or developing technology, that may be applied for example, in the cleaning, etching, and deposition of materials on very large scale integrated circuit (VLSI) wafers. Such applications are typical of other processing technologies requiring the capability of achieving submicron feature dimensions on these wafers, substantial processing rates, and the capability of uniformly processing large specimens. Any such technology must operate in a reproducible manner and be implemented in a cost-effective method. The general characteristics of these technologies are believed to be well understood by those skilled in the art and are accordingly not discussed in substantive detail herein.
Although the ECR plasma source of the present invention is contemplated for use in a wide variety of chemical vapor deposition or etching applications, the general parameters of applications are not a portion of the invention. Certain considerations in such applications are briefly summarized below in order to facilitate a better understanding the microwave applicator of the present invention.
ECR plasma sources such as those provided by the present invention and the prior art discussed below employ magnetic fields and microwave power to create chemically active plasmas over a range of gas pressures extending to very low values. Low-pressure operation may be desirable in order to permit the formation of anisotropic, highly directional streams of low-temperature ions, radicals, and other species of reactants which are uniform over transverse dimensions that are substantially larger than the specimen being processed.
In an ECR plasma source designed for plasma-enhanced chemical vapor deposition or reactive ion etching of VLSI wafers for example, a suitable mixture of gases is introduced into an evacuated chamber that is immersed in a suitably configured steady magnetic field and irradiated by electromagnetic radiation. The frequency of the radiation f.sub..mu. is selected to equal the electron gyrofrequency in a region of the steady magnetic field called the resonant interaction region. A resonance condition relates the strength of the steady magnetic field in the resonant interaction region, B.sub.res, to the frequency of the electromagnetic radiation; namely, f.sub..mu. =eB.sub.res /(2 .pi.m), where e is the magnitude of the electric charge and m is the mass of the electron.
Electrons in the resonant interaction region gain kinetic energy from the electromagnetic radiation, and if the radiated power and the gas pressure are suitably adjusted, the heated electrons can ionize the gas molecules to create a plasma. The plasma ions and electrons flow out of the resonant interaction region and impinge on the VLSI wafer, where the ions can deposit new materials, sputter existing materials, or participate in the etching of the existing film. If the plasma density is sufficiently high, the deposition, sputter, or etch rates can be substantially rapid. If the ion energies are sufficiently low, damage to the specimen being processed can be prevented, even if radio frequency (RF) bias of the specimen is employed. For etching narrow, deep, submicron-scale features, it is necessary for the ion trajectories to be highly directional. This is made possible by operating with an RF bias and at sufficiently low gas pressures, to ensure that the mean-free-path for ion scattering is longer than the length of the sheath region separating the specimen from the body of the plasma.
Additionally, if the temperature of the plasma ions is sufficiently low, and if there are no super-thermal groups of energetic ions, the specimen can be biased electrically, relative to the plasma interior for example, by applying RF power to the specimen. In this way, the specimen can be biased electrically to electrostatic potentials large enough to provide highly anisotropic processing, without exceeding a threshold ion energy above which excessive damage to the specimen may occur.
In order to process specimens of commercial interest, it is necessary to insure that the stream of plasma from the ECR source be uniform over transverse dimensions greater than 15 cm to 30 cm. The microwave applicator of the present invention addresses the need for large, uniform streams of low-temperature plasma, with high densities of ions and electrons in low-pressure mixtures of neutral gases.
In the prior art, one class of ECR plasma sources is generally referred to as a "Sumitomo source". The Sumitomo source is illustrated for example, by a plasma deposition apparatus of Matsuo et al., U.S. Pat. No. 4,401,054, issued Aug. 30, 1983, and further discussed in T. Ono, in Proceedings of the Tenth Symposium on IISIAT, 1986 Tokyo, p. 471; and T. Ono, M. Oda, C. Takahashi, and S. Matsuo, J. Vac. Technol. B4, 696 (1986).
In the apparatus disclosed in U.S. Pat. No. 4,401,054, to Matsuo et al., plasma flows toward the specimen along magnetic lines of force. It is very difficult to achieve a desired degree of spatial uniformity in the plasma density using the disclosed apparatus. Further, since electrons heated in the resonant interaction region flow directly toward the specimen along the magnetic lines of force, it is necessary to limit the applied microwave power in order to avoid the creation of unstable bursts of energetic electrons and associated groups of energetic ions that might damage the specimen being processed.
The physical processes responsible for the generation of these unstable bursts of electrons, together with accompanying super-thermal groups of energetic ions, are discussed in a first reference, a paper by B. H. Quon and R. A. Dandl entitled "Preferential electron-cyclotron heating of hot electrons and formation of overdense plasmas", Phys. Fluids B 1 (10) October 1989; a second reference, a paper by G. E. Guest, M. E. Fetzer, and R. A. Dandl entitled "Whistler-wave electron cyclotron heating in uniform and nonuniform magnetic fields", Phys. Fluids B 2 (6) June 1990; and a third reference, a paper by R. A. Dandl and G. E. Guest entitled "On the low-pressure mode transition in electron cyclotron heated plasmas", J. Vac. Sci. Technol. A 9 (6), November/December 1991.
Related prior art references include: U.S. Pat. No. 4,492,620, issued Jan. 8, 1985 to Matsuo et al. and entitled "Plasma Deposition Method and Apparatus" and U.S. Pat. No. 4,564,997, issued Jan. 21, 1986, to Matsuo et al. and entitled "Semiconductor Device and Manufacturing Process Thereof". The three preceding U.S. patents issued under assignment to Nippon Telegraph & Telephone Public Corporation. Additional related references that issued under common assignment include: U.S. Pat. No. 4,450,031, issued May 22, 1984, to Ono et al. and entitled "Ion Shower Apparatus"; U.S. Pat. No. 4,503,807, issued Mar. 12, 1985, to Nakayama et al. and entitled "Chemical Vapor Deposition Apparatus"; and U.S. Pat. No. 4,566,940, issued Jan. 28, 1986, to Itsumi et al. and entitled "Manufacturing Process for Semiconductor Integrated Circuits".
Another design of an ECR plasma source (of a very different fundamental nature) originated in France and is disclosed in U.S. Pat. No. 4,534,842, issued Aug. 13, 1985, to Arnal, et al. and entitled "Process and Device for Producing A Homogeneous Large-Volume Plasma of High Density and Low Electronic Temperature". In the disclosed process and device, plasma is accumulated in a large volume of a very low magnetic field strength, so as to enhance the spatial uniformity of the plasma. However, the resonant interaction region is localized to a small volume in the vicinity of metallic conducting antennas, that are placed close to a wall of a chamber. This approach of coupling the microwave power to the plasma provided limited efficiency for plasma generation and led to excessive generation of metallic impurities.
Various concepts for ion sources are disclosed in a number of U.S. patents that include: U.S. Pat. No. 3,571,734, issued Mar. 23, 1971; U.S. Pat. No. 3,774,001, issued Nov. 20, 1973; U.S. Pat. No. 3,790,787, issued Feb. 5, 1974; U.S. Pat. No. 4,417,178, issued Nov. 22, 1983; U.S. Pat. No. 4,638,216, issued Jan. 20, 1987; U.S. Pat. No. 3,500,077, issued Mar. 10, 1970; U.S. Pat. No. 3,582,849, issued Jun. 1, 1971; U.S. Pat. No. 3,660,715, issued May 2, 1972; U.S. Pat. No. 3,663,360, issued May 16, 1972; and U.S. Pat. No. 3,742,219, issued Jun. 26, 1973. The above cited prior art, including references cited therein, are incorporated herein by reference in order to insure an understanding of the background of ECR plasma sources and applications therefor.
U.S. Pat. No. 5,370,765 to Dandl, and references cited therein, discloses a method and apparatus where plasmas that are ideally suited to semiconductor processing applications can be created using an ECR plasma source, wherein a unique slotted-waveguide microwave applicator is matched to a novel magnetic configuration to provide spatially distributed, high-field launch of electron cyclotron heating power. Practical embodiments of the disclosed method and apparatus employ microwave power at relatively high frequencies so that the physical size of the applicator and the resulting plasma source match present day processing equipment designed for either 6 or 8 inch wafers. Readily available microwave power sources at frequencies near 6.2 GHz are utilized, together with high energy density permanent magnets, to create the requisite magnetic field strengths in excess of 2.2 kg, which is the cyclotron resonant magnetic field strength, B.sub.res, for a microwave frequency f.sub..mu. of 6.2 GHz.
The approach disclosed by Dandl was found to meet the need for an ECR plasma source capable of satisfying the requirements for a variety of applications. The disclosed method and apparatus is capable of producing large, uniform streams of quiescent, low-temperature plasmas with high densities of ions and electrons, especially in low-pressure mixtures of neutral gases so as to yield anistropic, highly directional plasma flow, as well as the desired composition of reactant species. Further, the disclosed approach can be implemented in a wide variety of configurations, including cylindrical and planar, as well as more generally shaped configurations. The chamber walls of the disclosed apparatus may be fabricated from fused quartz, alumina, stainless steel, nickel, or aluminum, for example, depending on the specific requirements of the process for which the plasma is to be employed.
However, it would be desirable to employ lower frequency microwave power, such as at the very common microwave frequency of 2.45 GHz. Employing a frequency of 2.45 GHz aids with achieving significant economic benefits that would result from using less expensive or lesser amounts, or both, of lower energy density permanent magnet materials. Further, a frequency of 2.45 GHz would enable the use of less expensive magnetron microwave sources and associated power supplies.
In order to operate at 2.45 GHz, the slotted-waveguide microwave applicator disclosed by Dandl would have to range from 5-8 in.sup.2 in cross-sectional area. A disadvantage of such an applicator would be that the applicator is substantially large and bulky, so that the associated magnetic field generating structure would be inefficient and generally unsuited to semiconductor processing applications as they are presently configured. An essential feature of the ECR plasma source disclosed by Dandl is a distributed microwave applicator that is suitably matched to a novel magnetic field configuration. A variant of the slotted-waveguide antenna disclosed by Dandl would be advantageous. In particular, it would be advantageous if the slotted-waveguide antenna could be replaced by a similarly functioning coaxial antenna, so that the size of the applicator could be reduced to a fraction of an inch in thickness, and the magnetic field generating structures could be made especially efficient as well as optimally effective in generating uniform plasmas.